The horizontal distance between panel rows:

Please measure this as the distance from the front of one panel to the next, not the space in between the rows (in other words, don't take the panel thickness into account here).

Note that the panel spacing is measured along the surface on which the panels are installed, even if it's tilted. So if the panels are installed on a rooftop, measure the spacing along the roof (not projected onto the ground below).

You can use any units for the panel spacing—feet, inches, meters, cm, furlongs, etc.—as long as you use the same units for all panel measurements.